Body mass index, or BMI, is a tool doctors use to determine if a person is overweight, and to what degree. It is a calculation based on height and weight that can provide a better estimate of total body fat than weight alone.
For most adults, a normal body mass index is between 18.5 to under 24.9.
Body mass index is an expression of the relationship between a person’s height and weight as a single number not dependent on frame size. BMI is calculated from the weight and square of the height: BMI = body weight (in kg) ÷ height (in meters) squared.
| Measurement | What It Means |
|---|---|
| BMI less than 18.5 | Below normal weight |
| BMI 18.5 to under 24.9 | Normal weight |
| BMI 25 to under 29.9 | Overweight |
| BMI 30 to under 34.9 | Class I Obesity |
| BMI 35 to under 39.9 | Class I Obesity |
| BMI 40 or greater | Class III Obesity |
In adults, body mass index does not correlate to age. BMI is used slightly differently in children, in that it is calculated the same way as it is for adults, but then the ranges are compared to percentiles for children of the same age, sex, height, and weight.
Are There Drawbacks of Using Body Mass Index?
Body mass index (BMI) is simply one measure healthcare professionals can use to assess whether a person is a healthy weight, but other factors must also be taken into account.
For example, in people who are muscular, such as bodybuilders, boxers, and other professional athletes, the body mass index may overestimate their body fat because muscle is denser than fat. This means an athletic person may be categorized as obese based solely on body mass index, but they are actually a healthy weight.
On the other hand, older people who have loss of muscle mass due to aging may have their body fat percentage underestimated.
Body mass index should not be used as a measurement for pregnant women.
From 
https://www.uptodate.com/contents/obesity-in-adults-prevalence-screening-and-evaluation?search=bmi&source=search_result&selectedTitle=2~150&usage_type=default&display_rank=2